How to append parent object from an external file including all the children?
I asked around and a user JA12 on #blender irc channel suggested me the following:


Open up the source scene, select the parent object in the Outliner with Ctrl+Left Click, that will select all levels of children as well
Use Ctrl+C (Copy) while those objects are selected, that will copy the selection to the buffer
Close the source scene and open up the destination scene
Press Ctrl+V (Paste)


Note: Copy Attributes Add-on prevents default copy action.